1. Field of the Invention
The invention relates to noise reduction. systems for induction motors, and more particularly baffled ducts that may be fitted to motor cooling vents in order to dampen windage noise propagation caused by rotating motor shafts and rotors.
2. Description of the Prior Art
Operating induction motors generate windage noise, caused by the rotating shaft, rotor and related rotating hardware. The windage noise propagates through motor cooling airflow paths that provide a heat transfer path out of the motor housing, including any external shrouds or other motor enclosures, via housing cooling vents. While the motor housing frame, external shrouds and other enclosures absorb or deaden some windage noise, the housing cooling vents enable noise propagation directly to surrounding ambient air. It is desirable to minimize noise exposure to workers who are located near operating motors, especially in the frequency range of 400-4000 Hz that has a greater impact on human hearing than higher or lower frequencies.
Known ways to minimize windage noise propagation from induction motors have included sound-deadening shrouds or other enclosures that surround the motor and become part of the motor housing. However, it is impractical to shroud larger motors of greater than 1000 horsepower due to their large size. In another known solution, motor housing cooling vents have been coupled to sound dampening ducts having generally parallel planar baffles of identical thickness and lateral spacing that are oriented parallel to the duct cooling airflow path. Lateral spacing of adjacent baffle plates less than the sound propagation wavelength of a given windage noise propagation frequency dampens the noise. While such identical baffle constructions dampen some relevant low frequency (large wavelength) noise, such known constructions do not provide noise dampening over a broad spectrum of noise frequencies. If such known baffle duct construction is modified to decrease spacing between baffles so as to dampen the relatively shorter wavelength of higher noise frequencies, overall cooling airflow performance of the cooling vent decreases to an unacceptable performance level. For example, adjacent baffle spacing of 5.5 inches (140 mm) is sufficient to dampen a 400 Hz noise frequency. Such relatively wide adjacent baffle spacing provides for sufficient air flow rate and volume into the motor cooling vent. However, a 4000 Hz noise frequency may require baffle parallel spacing of only 0.001 inch (0.028 mm) to dampen sufficiently that frequency. Laterally spacing all baffles with a gap of 0.001 inch would unduly restrict air flow into the motor cooling vent. Generally one skilled in the art would prefer a minimum baffle lateral spacing of no less than 0.5 inch (13 mm) to allow sufficient cooling air flow. Therefore uniformly spaced baffle dampers potentially sacrifice higher frequency noise attenuation when their shorter wavelengths pass between baffles set at the minimum baffle lateral spacing.
Also, known identical uniform baffle construction and spacing damping ducts provide a single solution for all applications, whereas different applications might benefit from baffle ducts constructed to meet the noise damping needs of a specific motor design or application for a motor design. Different motor constructions may have different noise propagation frequency signatures. Furthermore noise signatures may be changed by specific installations and field applications.
Thus, a need exists in the art for an induction motor noise reduction system capable of dampening a broad spectrum of windage noise frequencies propagating from motor cooling vents that does not unduly restrict cooling airflow to the motor.
Another need exists in the art for an induction motor noise reduction system capable of being tuned to dampen windage noise propagating from motor cooling vents of motors having differing noise propagation properties due to their specific construction traits or installation application.